Microlithography is used for the production of microstructured components such as for example integrated circuits or LCDs. The microlithography process is carried out in what is referred to as a projection exposure apparatus having an illumination system and a projection objective. The image of a mask (commonly referred to as a reticle) illuminated via the illumination system is in that case projected via the projective objective on to a substrate (for example a silicon wafer) which is coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection objective in order to transfer the mask structure on to the light-sensitive layer of the substrate.
To enhance imaging contrast, given polarization distributions can be specifically adjusted in the reticle plane in the illumination system. But, the intensity distribution in the plane of the wafer can undesirably change depending on the polarization distribution which is set in the illumination system, that is to say which is obtained in the object plane of the projection objective. This can be due to transmission properties of the projection objective that are polarization-dependent. The known effect of polarization-dependent transmission is referred to as transmission separation or ‘diattenuation’. The effect is caused by the polarization-dependent transmission of anti-reflection layers provided on the lenses (AR-layers) and highly reflecting layers present on some mirrors (HR-layers) in the projection objective. Thus as is known for an AR-layer Tp is greater than Ts, wherein Tp is the degree of transmission for the p-component with the vibration direction of the electrical field strength vector parallel to the plane of incidence and Ts denotes the degree of transmission for the s-component with the vibration direction of the electrical field strength vector perpendicular to the plane of incidence.
To demonstrate the problem, FIGS. 15a-c show diagrammatic views illustrating the position-dependent configuration, respectively obtained in the wafer plane of a projection objective 5, of the scanned intensity (curves A2, B2 and C2) depending on the polarization distribution set in the reticle plane or the object plane of the projection objective 5. In that respect, all three cases are based on the assumption of a homogeneous intensity distribution in the reticle plane (curves A1, B1 and C1), in which respect however unpolarized light is set in the reticle plane as shown in FIG. 15a, polarized light is set in the reticle plane, with a radial polarization distribution, as shown in FIG. 15b, and a tangential polarization distribution is set in the reticle plane as shown in FIG. 15c. 
The term ‘tangential polarization’ is used to denote a polarization distribution in which the vibration planes of the electrical field strength vectors of the individual, linearly polarized light beams are oriented approximately perpendicularly to the radius directed on to the optical axis. In contrast the term ‘radial polarization’ is used to denote a polarization distribution in which the vibration planes of the electrical field strength vectors of the individual, linearly polarized light beams are oriented approximately radially with respect to the optical axis.
A comparison of the curves A2, B2 and C2 in FIG. 15a, FIG. 15b and FIG. 15c shows that, in spite of the homogeneous intensity distribution respectively present in the reticle plane, a homogeneous intensity distribution is also afforded in the wafer plane only for the unpolarized illumination mode of FIG. 15a whereas the polarization-dependent transmission of the projection objective 5, as shown in FIGS. 15b and 15c, leads to intensity distributions which respectively locally vary in the wafer plane and which are also different from each other.